一、关键点解析

1、?template<typename _Tp1, typename = typename
?? ? ? ? ? std::enable_if<std::is_convertible<_Tp1, _Tp>::value>::type>
?? ?__weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) noexcept

      template<typename _Tp1, typename = typename
	       std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
	__weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
	: _M_refcount(__r._M_refcount)
        { _M_ptr = __r.lock().get(); }

? ? 如果std::is_convertible<_Tp1*, _Tp*>::value是false会发生编译错误。

2、lock函数

template<typename _Tp, _Lock_policy _Lp>
class __shared_ptr
{
...
      __shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t)
      : _M_refcount(__r._M_refcount, std::nothrow)
      {
	_M_ptr = _M_refcount._M_get_use_count() ? __r._M_ptr : nullptr;
      }
...
}



template<typename _Tp>
class shared_ptr : public __shared_ptr<_Tp>
{
...
     // This constructor is non-standard, it is used by weak_ptr::lock().
      shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t)
      : __shared_ptr<_Tp>(__r, std::nothrow) { }
...
}


  __shared_ptr<_Tp, _Lp>
      lock() const noexcept
      { return __shared_ptr<element_type, _Lp>(*this, std::nothrow); }

#include <memory>
#include<iostream>
int main()
{
	std::shared_ptr<int> ptr(new int(2));
//	std::weak_ptr<int> wptr= ptr;
//	ptr=NULL;
//	std::shared_ptr<int> ptr3=wptr.lock();
	std::weak_ptr<int> wptr= ptr;
    ptr=NULL;
    std::cout<<wptr.use_count()<<std::endl;
    std::shared_ptr<int> ptr3=wptr.lock();
	std::cout<<ptr3.get()<<std::endl;


	
	return 0;
 }

? ? ? ?lock函数返回weak_ptr对应的shared_ptr对象，但是如果所对应的shared_ptr对象的引用计数为0，返回shared_ptr对象的_M_ptr为nullptr。

  // Now that __weak_count is defined we can define this constructor:
  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::
    __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t)
    : _M_pi(__r._M_pi)
    {
      if (_M_pi != nullptr)
	if (!_M_pi->_M_add_ref_lock_nothrow())
	  _M_pi = nullptr;
    }


  template<>
    inline bool
    _Sp_counted_base<_S_atomic>::
    _M_add_ref_lock_nothrow()
    {
      // Perform lock-free add-if-not-zero operation.
      _Atomic_word __count = _M_get_use_count();
      do
	{
	  if (__count == 0)
	    return false;
	  // Replace the current counter value with the old value + 1, as
	  // long as it's not changed meanwhile.
	}
      while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
					  true, __ATOMIC_ACQ_REL,
					  __ATOMIC_RELAXED));
      return true;
    }

? __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t)构造函数中，__weak_count所对应的use_count为0，__shared_count中的?_M_pi 为?nullptr。

二、源码分析

  template<typename _Tp, _Lock_policy _Lp>
    class __weak_ptr
    {
    public:
      typedef _Tp element_type;

      constexpr __weak_ptr() noexcept
      : _M_ptr(0), _M_refcount()
      { }

      __weak_ptr(const __weak_ptr&) noexcept = default;
      __weak_ptr& operator=(const __weak_ptr&) noexcept = default;
      ~__weak_ptr() = default;

      // The "obvious" converting constructor implementation:
      //
      //  template<typename _Tp1>
      //    __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
      //    : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
      //    { }
      //
      // has a serious problem.
      //
      //  __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
      //  conversion may require access to *__r._M_ptr (virtual inheritance).
      //
      // It is not possible to avoid spurious access violations since
      // in multithreaded programs __r._M_ptr may be invalidated at any point.
      template<typename _Tp1, typename = typename
	       std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
	__weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
	: _M_refcount(__r._M_refcount)
        { _M_ptr = __r.lock().get(); }

      template<typename _Tp1, typename = typename
	       std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
	__weak_ptr(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
	{ }

      template<typename _Tp1>
	__weak_ptr&
	operator=(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
	{
	  _M_ptr = __r.lock().get();
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      template<typename _Tp1>
	__weak_ptr&
	operator=(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
	{
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      __shared_ptr<_Tp, _Lp>
      lock() const noexcept
      { return __shared_ptr<element_type, _Lp>(*this, std::nothrow); }

      long
      use_count() const noexcept
      { return _M_refcount._M_get_use_count(); }

      bool
      expired() const noexcept
      { return _M_refcount._M_get_use_count() == 0; }

      template<typename _Tp1>
	bool
	owner_before(const __shared_ptr<_Tp1, _Lp>& __rhs) const
	{ return _M_refcount._M_less(__rhs._M_refcount); }

      template<typename _Tp1>
	bool
	owner_before(const __weak_ptr<_Tp1, _Lp>& __rhs) const
	{ return _M_refcount._M_less(__rhs._M_refcount); }

      void
      reset() noexcept
      { __weak_ptr().swap(*this); }

      void
      swap(__weak_ptr& __s) noexcept
      {
	std::swap(_M_ptr, __s._M_ptr);
	_M_refcount._M_swap(__s._M_refcount);
      }

    private:
      // Used by __enable_shared_from_this.
      void
      _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
      {
	_M_ptr = __ptr;
	_M_refcount = __refcount;
      }

      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
      friend class __enable_shared_from_this<_Tp, _Lp>;
      friend class enable_shared_from_this<_Tp>;

      _Tp*	 	 _M_ptr;         // Contained pointer.
      __weak_count<_Lp>  _M_refcount;    // Reference counter.
    };

  // 20.7.2.3.6 weak_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) noexcept
    { __a.swap(__b); }